Mild and efficient rhodium-catalyzed deoxygenation of ketones to alkanes

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(1)Mild and eﬀicient rhodium-catalyzed deoxygenation of ketones to alkanes Gilles Argouarch. To cite this version: Gilles Argouarch. Mild and eﬀicient rhodium-catalyzed deoxygenation of ketones to alkanes. New Journal of Chemistry, Royal Society of Chemistry, 2019, 43 (28), pp.11041-11044. �10.1039/c9nj02954k�. �hal-02278412�. HAL Id: hal-02278412 https://hal-univ-rennes1.archives-ouvertes.fr/hal-02278412 Submitted on 23 Sep 2019. HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés..

(2) Please do not adjust margins. COMMUNICATION Mild and efficient rhodium-catalyzed deoxygenation of ketones to alkanes. molybdenum complexes were used as well and the deoxygenation of ketones was achieved by PhSiH3 with catalytic amounts of [MoCl2O2(H2O)2],8 and [ReOCl3(SMe2)(OPPh3)] or [ReOCl3(PPh3)2].9 Finally, an oxazolinylborate-coordinated rhodium silylene, highly active in the deoxygenation of esters to ethers, was briefly tested on ketones but led to the formation of alkanes with modest yields.10 Herein we disclose that the commercially available rhodium(I) dimeric complex [Rh(-Cl)(CO)2]2 constitutes a simple and efficient catalyst for the mild deoxygenation of aromatic ketones under hydrosilylation conditions. Although this wellknown neutral complex has been used previously in very important catalytic transformations,11 to the best of our knowledge, this represents its first application in the reduction of carbonyl compounds. Optimization experiments were carried out with 4acetylbiphenyl (1a) as the model substrate (Scheme 1). Various solvents and silanes were tested with a catalyst loading of 1 mol%. After reaction and acidic hydrolysis, the conversion of 1a and the proportions of alcohol 2a to hydrocarbon 3a were determined by 1H NMR (Table 1).. an us. A new and simple method for the deoxygenation of ketones to alkanes is presented. Most substrates are reduced under mild conditions by triethylsilane in the presence of catalytic amounts of [Rh(-Cl)(CO)2]2. This system selectively provides the methylene hydrocarbons in good to excellent yields starting from acetophenones and diaryl ketones. A rapid examination of the reaction pathway suggests that the ketone is first converted in alcohol which then undergoes hydrogenolysis to give the alkane.. cr ip t. Gilles Argouarch*. Ac. ce. pt. ed. m. Over a century ago the reactions of Clemmensen and WolffKishner opened the way to the transformation of ketones to alkanes through carbonyl-to-methylene complete reduction. Unfortunately, these methodologies were suffering from harsh reaction conditions making them incompatible to a large panel of functional groups. In the search for mild and less toxic reactions that reduce ketones into methylenes, later advances were made by the use of metal-hydride reagents or silanes with Lewis or Brønsted acids, even if some limitations remained regarding chemoselectivity.1 More recently, deoxygenation of ketones was also accomplished by transition-metal catalysis. In this field, a limited number of catalytic systems based on the use of molecular hydrogen as the reducing agent were reported and proved to be enough selective to overcome the hydrogenation competitive pathway in the course of the reaction.2 Even more rare are the metal-based catalysts employing a hydrosilane as the hydride source that allowed this deoxygenation reaction, in place of the more classical hydrosilylation reduction. Palladium species were first introduced in 2008 with the combination [PdCl2]/HSiEt3,3 and subsequent improvements were made by using [Pd(OAc)2],4 [PdCl2],5 or even Pd/C6 in the presence of polymethylhydrosiloxane (PMHS). Another method relying on the system [FeCl3]/PMHS and which necessitated a microwave irradiation was also reported. 7 High-valent rhenium and. Univ Rennes, CNRS, ISCR - UMR 6226, F-35000 Rennes, France. E-mail: gilles.argouarch@univ-rennes1.fr † Electronic Supplementary Information (ESI) available: experimental details and analytical data of the products. See DOI: 10.1039/x0xx00000x. Scheme 1 Reduction of 1a under various conditions. When the reduction of 1a was carried out with Ph2SiH2 and [Rh(-Cl)(CO)2]2, good conversions ranging from 89% to 95% were achieved in acetonitrile, toluene, THF, and DCM, and alcohol 2a was obtained as the major product (Table 1, entries 1-4). At that stage, the deoxygenated product 3a was formed. Please do not adjust margins.

(3) Please do not adjust margins COMMUNICATION. Table 1 Optimization experiments of the Rh-catalyzed deoxygenation of 4-acetylbiphenyl (1a) Silane. Solvent. Conv. (%)a. 2a/3a (%)a. 1. Ph2SiH2. MeCN. 89. 99/1. 2. Ph2SiH2. toluene. 94. 99/1. 3. Ph2SiH2. THF. 95. 95/5. 4. Ph2SiH2. DCM. 94. 99/1. 5. PMHS. DCM. 96. 92/8. 6. PhSiH3. DCM. >99. 72/28. 7. HSiEt3. DCM. >99. 1/99. Determined by. 1H. NMR of the crude products.. Table 2 Scope and limitations deoxygenation of ketonesa Entry. Ketone. 1. 2. 3. ed. pt. ce. HSiEt3. of. Product. the. Rh-catalyzed Conv. (Yield)b. >99 (84). HSiEt3. >99 (83). HSiEt3. >99. 4. HSiEt3. >99. 5. HSiEt3. >99 (63). 6. HSiEt3. >99 (67). 7. HSiEt3. >99 (62). 8. HSiEt3. >99. 9. HSiEt3. 78. 10. HSiEt3. 85 (71). 11. PhSiH3. >99 (88). 12. PMHS. >99. 13. HSiEt3. 0. 14. PhSiH3. >99 (94). 15. HSiEt3. >99. 16. PhSiH3. >99 (59). 17. HSiEt3. >99 (69). 18. HSiEt3. 0. m. The optimized reaction conditions were next presented to a variety of substrates (Table 2). Ketones 1a-r were thus treated with 1 mol% catalyst and a two-fold amount of HSiEt3 in DCM for one day. Overall, 1H NMR analysis of the crude samples after reaction revealed that no hydrosilylation adduct (alkoxysilane) was present or only in trace amounts. Consequently work-up became useless and the samples could be simply evaporated and subjected to chromatography. For electron-neutral ketones 1a-d, the corresponding arenes 3a-d were formed quantitatively and high isolated yields were obtained for 3a and 3b (Table 2, entries 1-4). The para and ortho isomers of methylacetophenone 1e and 1f gave also good rates of formation of products 3e and 3f which were isolated in 63% and 67% yields, respectively (Table 2, entries 5 and 6). These moderate yields were in part attributable to the low polarity of such compounds making their separation from the by-products of the silane sometimes delicate. This method was also tested on more encumbered substrates such as ketones 1g-i (Table 2, entries 7-9). Although a complete conversion was observed with -tetralone (1g) and cyclohexyl phenyl ketone (1h), isobutyrophenone (1i) only produced 78% of hydrocarbon 3i. Interestingly, this method could also be applied to ferrocene derivatives (Table 2, entries 10-14). With acetylferrocene (1j), an incomplete conversion of 85% was obtained when HSiEt 3 was used as the hydride source but fortunately PhSiH3 and PMHS afforded ethylferrocene (3j) in quantitative form. This contrasts from the results gathered with the model substrate 1a for which HSiEt3 was the most efficient silane to achieve deoxygenation and implies that the selectivity of this protocol is. Ac. Silane. an us. a. Entry. not restricted to one silane but is rather dependent on the two reactants in presence. Similarly, in the case of benzoylferrocene (1k), no reaction occured with HSiEt3 whereas PhSiH3 gave 3k in 94% isolated yield.. cr ip t. only in negligible amounts (1-5%). By using PMHS in DCM, the reactivity was maintained with an increase in the amount of 3a corresponding to 8% of the product (Table 1, entry 5). The more reactive silane PhSiH3 gave full conversion of the ketone and a significant change in the outcome of the reaction which gave 28% of 3a versus 72% of 2a (Table 1, entry 6). Finally, a switch of the selectivity in favor of the deoxygenation pathway was fully reached with a combination between the Rh(I) complex and the monohydrosilane HSiEt3 in DCM as 3a was virtually the only product arising from the complete reduction of 1a by this way (Table 1, entry 7). Under these conditions, attempts to decrease the catalyst loading turned out to be detrimental to the conversion of ketone 1a.. Please do not adjust margins.

(4) Please do not adjust margins COMMUNICATION. HSiEt3. >99. 20. HSiEt3. >99. 21. HSiEt3. 20. 22. HSiEt3. >99 (77). a. Typical conditions: ketone (1 mmol), silane (2 mmol), [Rh(Cl)(CO)2]2 (1 mol%), DCM (3 mL), 0 °C to RT, 24 h. b Conversions (%) determined according to 1H NMR spectroscopy of the crude products, isolated yields (%) are given into brackets.. an us. In the screening of ketones with significant electronic effects, some limitations were apparent (Table 2, entries 15-21). While methoxy groups on compounds 1l and 1m had no effect on the reactions affording alkylated anisoles 3l and 3m with good yields, the diethylamino moiety within the electron-rich ketone 1n completely inhibited the deoxygenation. Likewise, halogenated ketones 1o and 1p participated well in the reaction but a severe limitation were encountered in the case of the more challenging electron-poor 2,2,2-trifluoroacetophenone (1q) which gave a low conversion of 20%. In addition, as a typical diaryl ketone, benzophenone (1r) was also nicely converted to arene 3r in 77% isolated yield (Table 2, entry 22). The Rh/HSiEt3 system was further extended to miscellaneous substrates (Scheme 2). The phenolic ketone 1s was engaged in the reaction, and despite its full consumption, 4-ethylphenol (3s) was isolated only in 45% yield because triethylsilyl ethers 4 and 5 were formed concomitantly. This competition between deoxygenation and O-silylation was not surprising since rhodium complexes are known to catalyze the dehydrogenative silylation of alcohols and phenols with hydrosilanes. 12. likely the result of a 1,4-hydrosilylation of the starting enone followed by decomposition of the silyl enol ether intermediate, as reported elsewhere with rhodium,13 rather than a hydrogenation of the C=C bond with H2 arising from silane dehydrodimerization or hydrolysis.14 Similarly, flavone 1u was nicely converted into flavanone 7 which was isolated in 53% yield. Some experiments on various aldehydes and aliphatic ketones were also run. After hydrolysis, only alcohols were obtained. For instance, the reduction of 1,3-diphenylacetone (1v) led to the clean formation of 1,3-diphenylpropan-2-ol (2v) (Scheme 3). Keto-esters followed the same trend according to which ethyl phenylglyoxalate (1w) was fully transformed into ethyl mandelate (2w).. cr ip t. 19. Scheme 3 Rh-catalyzed hydrosilylation of compounds 1v and 1w.. Ac. ce. pt. ed. m. In the literature,4-9 although the mechanistic details of the transition-metal catalyzed deoxygenation of ketones in the presence of silanes are not yet clear, it was established that this reductive process proceeds in two steps, where the ketone is first reduced to an alcohol which in turn undergoes hydrogenolysis to form the alkylated product. To assess this with rhodium, control experiments were conducted (Scheme 4).. Scheme 2 Rh-catalyzed reduction of compounds 1s-u. The reaction of the ,-unsaturated ketone 1t went to completion and gave product 6 in good yield (76%). This was. Scheme 4 Control experiments on the deoxygenation process and generalization of the Rh-catalyzed hydrogenolysis of benzylic alcohols.. Please do not adjust margins.

(5) Please do not adjust margins COMMUNICATION. 7 8 9 10 11. 12. A. Volkov, K. P. J. Gustafson, C.-W. Tai, O. Verho, J.-E. Bäckvall and H. Adolfsson, Angew. Chem. Int. Ed., 2015, 54, 5122. C. Dal Zotto, D. Virieux and J.-M. Campagne, Synlett, 2009, 276. S. C. A. Sousa, T. A. Fernandes and A. C. Fernandes, Eur. J. Org. Chem., 2016, 3109. T. A. Fernandes, J. R. Bernardo and A. C. Fernandes, ChemCatChem, 2015, 7, 1177. S. Xu, J. S. Boschen, A. Biswas, T. Kobayashi, M. Pruski, T. L. Windus and A. D. Sadow, Dalton Trans., 2015, 44, 15897. For recent examples see: a) W. Song, N. Zheng, M. Li, K. Ullah and Y. Zheng, Adv. Synth. Catal., 2018, 360, 2429; b) N. Zheng, W. Song, T. Zhang, M. Li, Y. Zheng and L. Chen, J. Org. Chem., 2018, 83, 6210; c) Y. Kawaguchi, K. Yabushita and C. Mukai, Angew. Chem. Int. Ed., 2018, 57, 4707; d) Y. Kawaguchi, A. Nagata, K. Kurokawa, H. Yokosawa and C. Mukai, Chem. Eur. J., 2018, 24, 6538; e) C.-H. Liu and Z.-X. Yu, Angew. Chem. Int. Ed., 2017, 56, 8667; f) X. Li, J. Pan, H. Wu and N. Jiao, Chem. Sci., 2017, 8, 6266; g) Y. Liao, Q. Lu, G. Chen, Y. Yu, C. Li and X. Huang, ACS Catal., 2017, 7, 7529. a) I. Ojima, T. Kogure, M. Nihonyanagi, H. Kono and S. Inaba, Chem. Lett., 1973, 501; b) R. J. P. Corriu and J. J. E. Moreau, J. Organomet. Chem., 1976, 120, 337; c) M. P. Doyle, K. G. High, V. Bagheri, R. J. Pieters, P. J. Lewis and M. M. Pearson, J. Org. Chem., 1990, 55, 6082. Representative examples: a) G. Onodera, R. Hachisuka, T. Noguchi, H. Miura, T. Hashimoto and R. Takeuchi, Tetrahedron Lett., 2014, 55, 310; b) H. Sato, T. Fujihara, Y. Obora, M. Tokunaga, J. Kiyosu and Y. Tsuji, Chem. Commun., 2007, 269; c) L. V. Dinh and J. A. Gladysz, New J. Chem., 2005, 29, 173; d) G. Rivera and R. H. Crabtree, J. Mol. Catal. A:Chem., 2004, 222, 59. Several attempts to reduce styrene derivatives with the present system were unsuccessful.. 13. 14. Conflicts of interest. m. There are no conflicts to declare.. Acknowledgements. pt. Ac. 2. a) N. Sakai, K. Nagasawa, R. Ikeda, Y. Nakaike and T. Konakahara, Tetrahedron Lett., 2011, 52, 3133; b) J. Choi and Y. Kang, Bull. Korean Chem. Soc., 2005, 26, 343; c) T. Miyai, M. Ueba and A. Baba, Synlett, 1999, 182. Representative examples: a) G. La Sorella, L. Sperni, P. Canton, L. Coletti, F. Fabris, G. Strukul and A. Scarso, J. Org. Chem., 2018, 83, 7438; b) C. Schäfer, C. J. Ellstrom, H. Cho and B. Török, Green Chem., 2017, 19, 1230; c) D. M. Grainger, A. Zanotti-Gerosa, K. P. Cole, D. Mitchell, S. A. May, P. M. Pollock and J. R. Calvin, ChemCatChem, 2013, 5, 1205; d) L. Zhou, Z. Liu, Y. Liu, Y. Zhang and J. Wang, Tetrahedron, 2013, 69, 6083; e) M. Cano, A. M. Benito, W. K. Maser and E. P. Urriolabeitia, New J. Chem., 2013, 37, 1968; f) J. Hermannsdörfer and R. Kempe, Chem. Eur. J., 2011, 17, 8071; g) C. Van Doorslaer, J. Wahlen, P. G. N. Mertens, B. Thijs, P. Nockemann, K. Binnemans and D. E. De Vos, ChemSusChem, 2008, 1, 997. M. Mirza-Aghayan, R. Boukherroub and M. Rahimifard, J. Organomet. Chem., 2008, 693, 3567. R. J. Rahaim Jr. and R. E. Maleczka Jr., Org. Lett., 2011, 13, 584. a) H. Wang, L. Li, X.-F. Bai, J.-Y. Shang, K.-F. Yang and L.-W. Xu, Adv. Synth. Catal., 2013, 355, 341; b) H. Wang, L. Li, X.-F. Bai, W.-H. Deng, Z.-J. Zheng, K.-F. Yang and L.-W. Xu, Green Chem., 2013, 15, 2349.. ce. 1. ed. Université de Rennes 1 and CNRS are acknowledged for financial supports.. Notes and references. 3 4 5. cr ip t. 6. an us. When the benzylic alcohol 2a was engaged in the deoxygenation reaction, it was successfully converted to arene 3a which was isolated in 88% yield. On the contrary, no reaction occured starting from the trimethylsilyl ether 8. Thus, it can be postulated that the mechanism of this reaction also involves the release of an alcohol and that this latter probably doesn’t originate from a hydrosilylation adduct of type 8 that remained inert under the reaction conditions. Finally, the applicability of the hydrogenolysis of benzylic alcohols by the system [Rh(-Cl)(CO)2]2/HSiEt3 was briefly extended (Scheme 4). Besides the model compound 2a, full conversion of alcohols 2l and 2r also happened, and products 3l and 3r were isolated in 74% and 70% yields, respectively. In conclusion, this work has provided an original and straightforward method for the deoxygenation of ketones and has drawn light on the catalytic properties of the readily available rhodium complex [Rh(-Cl)(CO)2]2 in the reduction of carbonyl compounds. Herein, this precatalyst was able without the aid of any additives to convert the C=O group of a series of ketones into methylenes in the presence of weak reducing agents such as HSiEt3. This system operating under mild conditions and in non protic media was also efficient in the hydrogenolysis of benzylic alcohols. Other interesting uses of this catalyst in reductive reactions will be reported in due course.. Please do not adjust margins.

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